The genus notothylas Sull. in Thailand is reviewed, based on herbarium specimens and field surveys. Eight species of the genus are recognized and two of them, n. pandei Udar et V. Chandra and n. yunannensis T. Peng et R.L. zhu, are newly recorded in the country. The new species notothylas frahmii Chantanaorr. is described and illustrated. A key to species, brief descriptions and selected illustrations for the species of notothylas of Thailand are provided.